Impedance compensation

ABSTRACT

The invention relates to a method for operating a power converter ( 1 ), preferably a solar inverter, for feeding a load current (I load ) into a grid ( 3 ). The method includes the steps of determining the load current (I load ), monitoring an output voltage (U out ) of the power converter ( 1 ), and controlling the load current (I load ), to avoid that the output voltage (U out ) exceeds a voltage limit (U out,lim ), wherein the voltage limit (U out,lim ) is adjusted in dependency of the load current (I load ). Furthermore the invention relates to a corresponding power converter ( 1 ) for connecting to a grid ( 3 ), preferably it relates to a solar power converter. This power converter ( 1 ) includes a load current sensor ( 142 ), an output voltage sensor ( 143 ) and a load current controller ( 141 ).

TECHNICAL FIELD

The invention relates to a method for operating a power converter,preferably a solar inverter, for feeding a load current into a grid. Themethod includes the steps of determining the load current,

-   -   monitoring an output voltage of the power converter, and        controlling the load current to avoid that the output voltage        exceeds a voltage limit.

Furthermore the invention relates to a corresponding power converter forconnecting to a grid, preferably it relates to a solar power converter.This power converter includes a load current sensor, an output voltagesensor and a load current controller.

BACKGROUND ART

State of the art power converters feeding a load current into a gridusually have a fix voltage limit for a maximally permissible operationvoltage in order to protect the connected grid as well as the connectionbetween the power converter and the grid from overvoltage. When feedingcurrent into the grid the power converter's output voltage is alwayshigher than the voltage at the grid connection point due to lineimpedance. The output voltage may exceed the voltage limit while thegrid voltage still being in an acceptable level. In this case the powerconverter will shut down and not deliver its nominal power. One knownmethod to circumvent this problem is the use of line impedancecompensation systems.

The US2013/0033103 A1 (M C. Junkin et al.) discloses systems and methodsfor impedance compensation in subsea power distribution systems. Thesesystems and methods include a plurality of impedance compensationdevices comprising passive elements as resistors, diodes, capacitors andinductors, a controller to control the operation of those powercompensation devices and detectors to detect especially electricallyparameters, associated with the subsea power distribution system.

The disclosed systems and methods according to the US2013/0033103 allowfor eliminating the effect of line impedances, however additionalhardware is required.

SUMMARY OF THE INVENTION

It is the object of the invention to create a method pertaining to thetechnical field initially mentioned, which allows for the efficient useof the available output power of the power converter while avoidingadditional hardware costs.

The solution of the invention is specified by the features of claim 1.According to the invention the voltage limit is adjusted in dependencyof the load current.

The power converter monitors the output voltage and controls the loadcurrent such that the output voltage does not exceed the voltage limit.Herein the notion controlling may include a switching off of the loadcurrent and/or the power converter but also controlling a load currentsuch that it follows a reference value in general or a load currentreference value in particular. For this purpose a current controller isprovided which usually includes a feedback control of the load currentby manipulating an actuating means, as for instance a PWM modulator. Theload current reference value can also be zero Amperes (0 A).

Such a method has the advantage that different output voltage limits canbe set in dependency of the load situation of the power converter. Theadvantage is that the power converter is able to exploit its powercapacity efficiently in a wider operating range and is less dependent onthe quality of the grid connection. In addition no additional hardwareis required.

The adjustment of the voltage limit can be done in various ways, forexample by the step of defining a look up table attributing a voltagelimit for each load current value and the step of adjusting the voltagelimit for a given load current according to the look up table.

A special case of this embodiment is given when the adjusted voltagelimit is defined to be constant for all load current values. In anotherembodiment a two dimensional look up table may be defined, alsocomprising the grid voltage as an independent variable. This embodimentalso comprises the step of determining the actual grid voltage andadjusting the voltage limit according to the look up table.

In a preferred embodiment the adjustment of the voltage limit comprisesthe steps of determining a voltage correction value and adding thevoltage correction value to a grid voltage limit. The grid voltage,usually maintained by the utility fluctuates in a defined toleranceband, the upper limit of this tolerance band is designated as the gridvoltage limit. The advantage of this embodiment is that the voltagelimit is not dependent on the actual grid voltage, and monitoring of thegrid voltage is not necessary. So no additional voltage sensor isrequired, moreover, the correction term can more easily be defined.

In a further preferred embodiment of the invention the voltagecorrection value is determined by estimating a voltage drop due to aline impedance of a line between the power converter and the grid. Theline impedance between the power converter output and the gridconnection point may also be nonlinear in dependency of the loadcurrent. The finding that the correction value is related to the voltagedrop due to the line impedance simplifies the determination of thevoltage correction value. The estimation of the voltage drop can also beachieved by deriving models of the line impedance describing therelation between the load current and the voltage drop and/or by usingidentification methods involving measurements of the physical parametersund then by applying the load current to the model. There are manymethods known in the state of the art which can be used to get a modelof the line impedance. A well-known method is for instance the method oflinear regression in the case of a linear line impedance.

The line impedance and per consequence the voltage drop depends on thenetwork frequency and can therefore vary with frequency variation of thesystem voltage. It can also be time variant because of temperaturefluctuations, changes of the state of the transmission line, forinstance by switching on or off components within the path between theoutput and the grid connection point. All these changes can also beconsidered in the step of estimating the voltage drop. If state changesare considered the power converter may switch between different modelsstored in its memory in order to estimate the voltage drop.

By estimating the voltage drop due to the line impedance the influenceof the line impedance on the overvoltage of the output voltage can bealmost completely or at least to a large extent compensated by adjustingthe voltage limit.

In a further preferred embodiment the method of estimating the voltagedrop includes the steps of injecting test currents into the line,measuring the output voltage of the power converter for each testcurrent and identifying the line impedance. The power converter has themeans for injecting the test currents and to measure the output voltage.Those values are sufficient to identify the line impedance as long asthe grid voltage is stable during the measurement. The voltage drop canbe determined by applying the load current to the line impedance andmeasuring the output voltage. If the power converter is a DC-toAC or anAC-to-AC power converter the test currents are preferably alternatingcurrents which are constant in amplitude and frequency. The frequency ofthe alternating currents is preferably chosen to be similar to theutility frequency of the grid.

In a further embodiment the power converter is an AC-DC or a DC-DC powerconverter and the test currents are preferable DC-currents.

In a further embodiment of the invention at least two tuples of theoutput voltage and load current are measured during ordinary operationof the power converter and the line impedance is estimated from thatdata.

In another method the grid voltage is measured in addition to the outputvoltage when injecting the test current. Thus the voltage drop can bedirectly measured as difference between the output voltage and thevoltage drop and only one measurement point respectively one tuple ofthe output voltage and the load current is required in order to identifythe line impedance.

In another embodiment non constant respectively time varying testcurrents are injected into the grid. It might be advantageous forexample to inject test currents which follow pseudo-binary randomsignals or sine-sweep signals in order to gain also the phaseinformation of the line impedance in an efficient way.

The identification process typically is executed before activating thevoltage limit adjustment according to the invention or duringcommissioning of the adaption method. It should be repeated every timewhen the customers installation is subjected to alterations which couldcause a modification of the line impedance, for instance afterinstallation of new electrical devices in the network. Also whenswitching on or off of a device in the network, the line impedance couldchange. In such a case it might be considered to store the results ofthe identification process in a memory of the power converter and toretrieve the impedance value depending on the state of the network.

In another embodiment the estimation process will be executed at a giventime interval either manually or automatically. A timer could be used tosupervise the time interval if the estimation process has to be startedmanually. If the time interval has been exceeded the adjustment of thevoltage limit could be switched off automatically. On the one hand thelength of the time interval preferably has to be chosen in accordance tothe frequency of changes respectively modifications of the customerinstallation. On the other hand it might be adapted to a periodicity ofthe provided input power, for instance to the daily changes of sunactivity in the case of a solar inverter and/or the load of the grid,which is also submitted to daily fluctuations.

In a further preferred embodiment the voltage drop is estimated aslinear dependent on the load current. This signifies that also the lineimpedance is linear which is in most cases sufficient to estimate thevoltage drop due to the line impedance. The advantage is that theidentification of the line impedance and also the estimation of thevoltage drop is very simple and can be performed very efficiently.

In a further embodiment at least one test current is zero. When a testcurrent is zero, respective output voltage represents the open circuitvoltage. The open circuit voltage corresponds to the grid voltage as theoutput voltage for this test current depends only on the grid voltage.If more than one test current equal to zero are injected, it allows todetect grid voltage fluctuation between the measurements and tocompensate for that fluctuation or to invalidate the measurement.

In another embodiment all injected test currents are different fromzero. A linear line impedance can also be identified if at least twomeasurement points with two different current values are available, forinstance two test currents or one test current and a load current withdifferent amplitudes. Once the line impedance is identified, also thegrid voltage can be deduced by extrapolating the voltage drop for a loadcurrent of 0 A. An advantage of this embodiment is that the powerconverter can feed current into the grid also during the injection oftest currents. Another advantage of this embodiment is that a shortertime interval between the measurements can be achieved as the settlingtime for the different impressed currents will be reduced due to asmaller amplitude range between the load current and the test currents.

Advantageously the method comprises the step of injecting threeconsecutive test currents at equally spaced time intervals. The firsttest current will be set to zero, the second test current to a valuebetween 60% and 95% of the load current before the test, preferablybetween 75% and 85% of the load current and the third test current tozero again. The time interval between the injections of the testcurrents will be chosen in a range of 10 ms to 2500 ms, preferablybetween 100 ms and 500 ms.

The advantage of this method is that it comprises only three measurementpoints. As the open circuit voltage is measured twice, falsemeasurements due to grid variations can be detected and per consequencealso compensated. If the variation of both measurements exceeds apredefined limit, for example 0.5% of a nominal grid voltage, the testcan be defined as invalid. By taking equidistant time intervals aconstant grid voltage drift during measurement can be easily reduced bytaking the arithmetic mean value of both open-circuit voltagemeasurements. The time between the measurements should be long enough tocalculate a RMS-value of the output voltage respectively the testcurrent, but short enough, to keep the total measurement short. A shortmeasurement time also keeps the variation of the grid voltage short. Thesecond test current should be constant during the measurement whatimplies that it should be smaller than the actual current before thetest, e. g. 80%. Under normal load condition the load current is subjectto variations what is usually not desirable during the test. On theother hand, the test current should still be high enough in order tominimize the influence of measurement errors, as for instancemeasurement noise.

Alternatively also other measurement sequences could be selected. Forinstance only the open circuit voltage might be measured while thesecond measurement is taken during normal operation before themeasurement.

In an advantageous embodiment the voltage correction value is scaledsuch that the output voltage does not exceed a maximum output voltage ofthe power converter. By adjusting the maximum voltage in dependency ofthe load current in order to compensate for the voltage drop due to theline impedance, the output voltage may exceed the maximum output voltageof the power converter. The maximum output voltage of the powerconverter is among other things imposed by technical constraints ortechnical regulations.

The step of scaling may be linear or non-linear with the load current.Non-linear scaling may be advantageously considered if the estimatedvoltage drop is non-linear. Linearly scaling of the output voltage limithowever is very simple to implement and especially to parameterizeduring commissioning. Alternatively the voltage correction value mightbe defined by the minimum of the maximum output voltage and theestimated voltage drop due to the line impedance.

In a further preferred embodiment the step of controlling the loadcurrent includes the step of defining a limiting characteristics of theload current in dependency of the output voltage. Below a first outputvoltage level the limiting characteristics of the load current is equalto the nominal load current. Between a first output voltage level and asecond output voltage level this limiting characteristics has a negativegradient and covers a load current range between zero and the nominalload current. Above the second output voltage level, the limitingcharacteristics is zero. The first output voltage level is smaller thanthe second output voltage level and the second output voltage level isequal or smaller than the maximum output voltage of the power converter.The load current will be reduced if the output voltage exceeds the firstvoltage level, according to the limiting characteristics. Between thefirst and the second voltage level the load current will be furtherreduced with increasing output voltage, the limiting characteristicshaving a negative gradient between those voltages.

Above the second voltage level the current will be set to zero. It isalso possible to switch the power converter off if the second voltagelevel has been reached, or to switch it off, after a third voltagelevel, which is higher than the second voltage level, has been reached.

The limiting characteristics defines a limit of the load current infunction of the output voltage. Limiting the load current in function ofthe output voltage also has the effect of limiting the output power ofthe power converter in function of its output voltage. The advantage ofthis embodiment is that it provides a method of controlling the loadcurrent in function of the output voltage such that it preventsincreasing the output voltage due to the line impedance between theoutput of the power converter and the grid and prevents the powerconverter from shutting down or switching off. The converter will stillfeed current into the grid without exceeding the voltage limit, thoughthe load current will be reduced.

Another embodiment also provides a timer function which shuts down thepower converter if the output voltage exceeds the second voltage levelfor a predetermined time.

Alternatively the load current can be set to zero or even shut downimmediately after the first voltage level has been exceeded.

In a further preferred embodiment, the limiting characteristics betweenthe first output voltage level and the second output voltage level ischosen to be linear, respectively said negative gradient is constant.The advantage of this embodiment is that the implementation is verysimple.

In an alternative embodiment the negative gradient between the first andthe second voltage level may be variable. This may be advantageous inthe case of a non-linear line impedance.

Still, in another embodiment, the limiting characteristics vary in astepped respectively non-continuous manner between the first and thesecond input level instead of having a continuous negative gradient,wherein the levels of the steps are decreasing with the increasingoutput voltage. In a further embodiment of the invention, the limitingcharacteristics may very continuously with piece wise constantgradients.

The activation of the above mentioned load current reductionrespectively power reduction in function of the output voltage can alsobe activated independently from the adaption of the output voltagelimit.

Preferably, controlling the load current includes the step of adjustingthe load current stepwise to meet the limiting characteristics bygenerating intermediate load current reference values. The outputvoltage is a function of the load current, wherein the functioncomprises the grid voltage and the line impedance as parameters. For agiven grid voltage and a given line impedance, the relation between theload current and the output voltage defines a load currentcharacteristics which intersects the limiting characteristics in acurrent-voltage plane. The intersection point between bothcharacteristics represents a steady state point with a steady state loadcurrent and a steady state output voltage where the system will convergeto, if it is stable.

In a possible embodiment of the invention, the load current is adjusteddirectly to meet the limiting characteristics by measuring the gridvoltage, calculating the steady state load current which meets thelimiting characteristics, generating a load current reference valueequal to the steady state load current and controlling the load currentto reach the steady state load current. A disadvantage of this operationmethod is, that the actual grid voltage has to be measured which istypically very costly, if at all possible.

Usually the grid voltage is not available and therefore also the steadystate load current, respectively the steady state point, where the loadcurrent and the output voltage will converge to if the system is stable,are not known.

In addition, the adaption of the load current will not have an immediateeffect due to physical constraints respectively physical time constants.Examples of them are inductances in the load current path and time lagsin a control circuit of the power converter. Per consequence the loadcurrent reduction is preferably not applied instantaneously, but initerative steps. The step-size and the step-time have to be adapted tothe system, for instance to the gradient of the limiting characteristicsbetween the first and the second voltage level. In a preferredembodiment the step-by-step adaption of the load current is obtained byiteratively calculating an intermediate load current reference valueconsidering the actual load current value and the load current reductionfor the present output voltage according to the limitingcharacteristics. This embodiment allows reaching the steady state loadcurrent very fast under the condition that the system is well tuned. Ina further embodiment the intermediate load current reference value of anadjusting step is given by the load current reduction for the presentoutput voltage according to the limiting characteristics. Thisembodiment is very simple to implement but stability only can beachieved by adapting the gradient of the limiting characteristics. Inanother embodiment a digital filter is inserted. The digital filter isconsidering the load reference value of precedent steps and the loadcurrent reduction for the actual output voltage, such that anintermediate load reference value is generated and the adaption of theload reference values is smoother.

In a preferred embodiment the intermediate load current reference valuesI_(load,ref) are determined by the formula:

${I_{{load},{ref}} = {\left( {1 - \frac{{U_{out} \cdot I_{nom}} - {U_{100\%} \cdot I_{load}}}{{U_{out} \cdot I_{nom}} + {\left( {U_{0\%} - U_{100\%}} \right) \cdot I_{load}}}} \right) \cdot I_{nom}}},$

U_(0%) being the first output voltage level, U_(100%) being the secondoutput voltage level, I_(load) being the actual load current, U_(out)the actual output voltage, and I_(nom) the nominal load current of thepower converter. This formula is obtained by stepwise calculating thesteady state point under the assumption that the grid voltage is 0 V. Inthis preferred embodiment of the invention the load current referencevalues are generated such that the load current automatically convergestowards the steady state point without oscillations.

For a linear gradient of the limiting characteristics a stable behaviourwith no oscillations is achieved.

In a further preferred embodiment the first output voltage level and thesecond voltage level are adapted in dependency of the voltage limit.

If the load current is changing, the voltage limit will be adjusted. Inorder to take advantage of the adjustment of the voltage limit,preferably also the first and second voltage level have to be shiftedwith the voltage limit variation respectively the whole characteristicshas to be shifted with the voltage limit. In a further preferredembodiment the first voltage level is equal or in the range of theoutput voltage limit. The second voltage level has a constant distancefrom the first voltage level. The distance between the first and thesecond voltage level is chosen such that the gradient of the limitingcharacteristics is greater than the gradient of the load currentcharacteristics. If for instance the grid voltage increases, the effectof the load current reduction due to the limiting characteristics ismore important than the effect of the decreasing voltage limit, due to adecreasing load current. The load current will be reduced such that thepower converter will not switch off. The advantage of the embodiment isthat the power converter stays connected to the grid and does not haveto shut down and reconnect to the network.

In an alternative embodiment the first and the second voltage level arekept constant. In this case the load current reduction worksindependently of the voltage limit adjustment. The consequence is thatthe power converter may shut down for a certain load current range ifthe output voltage exceeds the output voltage limit.

Generally, the first or the second voltage level can be kept constant orthe first and the second voltage levels can be shifted by differentvalues in dependency of the voltage limit.

The invention also relates to a power converter for connecting to agrid, preferably a solar power converter, including a load currentsensor, an output voltage sensor and a load current controller forperforming the step of adjusting the voltage limit in dependency of theload current. The advantage of this embodiment is that the powerconverter is able to exploit its power capacity efficiently in a wideroperating range and is less dependent on the quality of the gridconnection. Other advantageous embodiments and combinations of featurescome out from the detailed description below and the totality of theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings used to explain the embodiments show:

FIG. 1 Schematic diagram of a power converter feeding a load currentinto a grid.

FIG. 2 Voltage characteristics over distance for a connection betweenthe power converter and the grid.

FIG. 3 Flow chart of an illustrative operation method according to theinvention.

FIG. 4 Voltage limit characteristics of an illustrative embodiment ofthe invention.

FIG. 5 A load current curve during injection of test currents accordingto the invention.

FIG. 6 An output voltage curve during injection of test currentsaccording to the example shown in FIG. 5.

FIG. 7 Example of a limiting characteristics of the load current as afunction of the output voltage, according to the present invention.

FIG. 8 Numerical example of determination of steady state point.

FIG. 9 Phase space diagram of load current and output voltageillustrating an embodiment of the operation method according to theinvention.

FIG. 10 Transient response of the load current according to the exampleshown in FIG. 9.

FIG. 11 Phase space diagram of load current and output voltageillustrating another embodiment of the operation method according to theinvention.

FIG. 12 Transient response of the load current according to the exampleshown in FIG. 11.

FIG. 13 Transient response of the load current of another preferredoperation method according to the present invention.

FIG. 14 Example of adaption of limiting characteristics to the loadcurrent.

FIG. 15 Schematic diagram of an embodiment of a power converter feedinga load according to the present invention.

In the figures, the same components are given the same referencesymbols.

Preferred Embodiments

FIG. 1 shows schematically a power converter 1 according to theinvention, feeding a load current I_(load) into a grid 3. In thisembodiment, the power converter 1 is fed by a DC voltage source 12 witha DC voltage U_(in) which is connected to the input 8 of the powerconverter 1, for instance a photovoltaic system. The power converter 1outputs an output voltage U_(out) to the grid 3, the output voltageU_(out) being an AC voltage. In another embodiment the power source alsocan be an AC source, for instance a wind turbine connected to agenerator. It is also possible that the power converter 1 feeds a DCcurrent into the grid 3. The line where the load current is fed into thegrid can be split in two parts: a dedicated lead wire 4, which connectsthe output 5 of the power converter 1 to a grid connection point 6 and agrid side connection network 7, which connects the grid connection point6 to the grid 3. The dedicated lead wire 4 usually belongs to theinstallation of the customer.

The impedance of the lead wire 4 and the line impedance of the grid sideconnection network 7 may be represented by lumped elements, namely aline impedance 9 and a grid side line impedance 10. At the gridconnection point 6 also other installations of the customer 11 may beconnected to the grid (dashed line). Due to the line impedance 9 and thegrid side line impedance 10, the output voltage U_(out) of the powerconverter is higher than a voltage U_(con) at the grid connection point6 and a grid voltage U_(grid) at the input of the grid in reference to aground 2 as common voltage reference. In this text the subscript “out”is used, when referring to a voltage at the output 5 of the powerconverter 1, the subscript “grid” when referring to a grid voltage whilethe subscript “con” is used when referring to a voltage at the gridconnection point. A further index “lim” is used to indicate a limitationof the output voltage at the output 5 of the power converter 1, “nom” ifthe output voltage is obtained when applying the nominal current. Incontrast to “lim” the subscript “limit” is used when speaking about asteady state condition. The subscript “max” designates a maximum of acertain voltage value.

FIG. 2 shows a typical voltage characteristics 20 over distance l for aconnection between the power converter output 5 and the grid 3 for agiven load current I_(load).

When feeding the load current I_(load) into the grid 3 the outputvoltage U_(out) of the power converter is always higher than the gridvoltage U_(grid). Starting at the power converter output 5 with theoutput voltage U_(out) the voltage drops to the voltage U_(con) at thegrid connection point 6. The voltage drops further from the gridconnection point 6 to the grid 3 to the grid voltage U_(grid). As thelead wire 4 usually has a poor line impedance 9 compared to the gridside connection network 7, the voltage drop U′_(drop) over the lead wire4 is typically significantly much more important than the voltage dropover the grid side connection network 7 and contributes therefore themajor part of the voltage drop U_(drop) between the output 5 of thepower converter 1 and the grid 3. Thus the line impedance 9 alsorepresents the major part of the impedance given by the sum of the lineimpedance 9 and the grid side line impedance 10. Though the voltage dropis piecewise linear between the power converter 1 and the grid 3, thevoltage drop may also be non-linear in dependency of the system. Also itmay be distributed over more than two piecewise linear subsections, asthe line itself can consist of different conductor sections, connectedin series and/or parallel.

Due to national regulations the voltage U_(con) at the grid connectionpoint 6 must not exceed the maximum allowed grid voltage U_(grid,max). Apurpose of this limit is to avoid that electrical loads in the publicnetwork are damaged due to over voltage. In Germany, for instance, thevoltage limit for low voltage networks with a nominal voltage of 230 Vis 10% over the nominal voltage, respectively U_(grid,max)=253 V.

In a first embodiment of the invention, the voltage limit of the powerconverter 1 U_(out,lim)(t) is adapted in dependency of the load currentI_(load)(t). FIG. 3 shows a flow chart 31 of the operation methodaccording to the invention.

The operation method enters at step S1 into a repetitive loop whichincludes the Steps S1-S6. In step S1 the run condition of the powerconverter 1 is checked. If the run condition is TRUE, the powerconverter stays in the iterative loop and continues with step S2.Otherwise, if the run condition is FALSE, it will continue with step S7and stop or switch off. When staying in the iterative loop, thesubsequent step S2 is executed, which includes the step of measuring theload current I_(load)(t). The measurement may for instance be performedby an integrated current sensor of the power converter 1. In the nextstep, S3, a voltage correction value is determined as a function of thetime dependent load current I_(load)(t). When the line impedance 9 (Z)is known, the load current depending voltage drop can be determined byapplying the term U_(drop)(t)=Z·I_(load)(t), where Z represents the lineimpedance 9. Then, in the succeeding step, S4, the voltage limitU_(out,lim) is adjusted by adding the load current depending voltagedrop U_(drop)(t) to the maximum grid voltage U_(grid,max):

U _(out,lim)(t)=U _(grid,max) +Z·I _(load)(t)

It is also possible, that the impedance Z is not linear in dependency ofthe load current I_(load). In this case the output voltage mightpreferably be adapted to

U _(out,lim)(t)=U _(grid,max) +Z(I _(load))·I _(load)(t),

considering the non-linearity of the impedance.

In the subsequent step, S5, the output voltage U_(out)(t) is monitoredin reference to the voltage limit U_(out,lim)(t). If the output voltageU_(out)(t) exceeds the voltage limit U_(out,lim)(t), the load currentI_(load) is adjusted in step S6 such that the output voltage does notexceed the voltage limit U_(out,lim)(t), before returning to step S1,otherwise step S1 is executed directly.

In a preferred embodiment according to the invention adjusting the loadcurrent in S6 may also include switching off or shutting down the powerconverter respectively setting the run condition to FALSE.

FIG. 4 shows a voltage limit characteristics 40 in function of the loadcurrent I_(load) of a further embodiment of the invention. The gridvoltage limit 41 and the maximum output voltage U_(out,max) 42 of thepower converter 1 are depicted in the graph by dashed lines. An unsealedvoltage limit characteristics 43 shows a curve of the previous mentionedembodiment of the invention where the voltage correction value isdetermined such that it compensates a measured line impedance, theunsealed voltage limit characteristics 43 having a gradient equal to theline impedance 9. In this case the maximum output voltage U_(out,max) 42of the power converter 1 would be exceeded for a load current I_(load)close to the nominal load current I_(nom).

In addition, the line impedance 9 might not be known precisely, or onlythe sum of the line impedance 9 and the grid side line impedance 10(FIG. 1). A preferred embodiment of the invention therefore provides thescaling of the voltage correction value such that the output voltagelimit U_(out,lim) 40 of the power converter 1 may not exceed the maximumoutput voltage U_(out,max) 42 of the power converter 1, for the entireload current range up to the nominal load current I_(nom).

A scaling factor c can be calculated by determining a maximum correctionvalue U_(diff), for the nominal load current I_(nom), such that themaximum output voltage is reached or the line impedance 9 is entirelycompensated.

The maximum correction value U_(diff) for the voltage limit U_(out,lim)40 is given by the minimum between the estimated voltage drop due to theline impedance 9 at the nominal load current I_(nom) and the differencebetween the maximum output voltage U_(out,max) 42 of the power converter1 and the maximum allowed grid voltage U_(grid,max). This yields for thescaling factor c defined by the ratio between the maximum correctionvalue U_(diff) and the voltage drop Z·I_(nom) due to the line impedanceZ at the nominal current I_(nom):

$c = {\frac{U_{diff}}{Z \cdot I_{nom}} = {\frac{\min \left( {{Z \cdot I_{nom}},{U_{{out},\max} - U_{{grid},\max}}} \right)}{Z \cdot I_{nom}}.}}$

Assuming an output voltage limit of the power converter 1 of 282V and agrid voltage limit of 253 V, a nominal load current of 100 A and a lineimpedance 9 of 0.5Ω. The maximum correction value U_(diff) equals then29V, yielding a scaling factor c of 0.58. The voltage limitcharacteristics 40 is than defined by the following relation:

U _(out,lim)(t)=U _(max,grid) +c·Z·I _(load)(t)

The operation method of adapting the voltage limit is preferablyactivated when facing problems with over voltage due to a poorinstallation, resulting for instance from long distances of the powerconverter 1 to the grid connection point or if the connection wire has alow cross section. But it can also be activated during the installationof the power converter 1. In another embodiment the function isactivated automatically after over voltage has been reached frequently,for instance more than once in 24 hours.

In a further embodiment of the invention the line impedance 9 isidentified by injecting AC-test currents by the power converter.

FIG. 5 shows a typical load current curve 50 of a preferred embodimentof the invention, where three consecutive test currents are injected bythe power converter 1 into the grid 3, at equally spaced time intervalsof 1 s.

FIG. 6 shows the corresponding output voltage signal curve 60.

When feeding test currents into the grid, the output voltage U_(out) ofthe power converter 1 changes. By comparing the output voltage U_(out)of the power converter 1 for different test currents, the totalimpedance between the power converter 1 and the grid 3, respectively thesum of the line impedance 9 and the grid side line impedance 10 (FIG.1), can be estimated. In most cases, the grid line impedance 10 onlycontributes a small part, respectively less than 10% to the measuredvoltage drop. In this case the voltage drop due to the grid side lineimpedance 10 can be neglected. Otherwise the contribution of the voltagedrop of the grid side line impedance 10 can be considered by thepreviously mentioned scaling factor c.

Before the start of the measurement, in the time interval −2 s<t<−1 s,the load current I_(load) varies in a range between 10 and 11 A. Thevariation of the load current I_(load) is among other things due tochanges of the supplied power at the input 8 of the power converter 1,for instance due to sun intensity changes if solar cells are connectedto the power converter 1 or due to wind speed variations in the casethat a wind turbine is connected to the power converter 1. With thestart of the test sequence, at the start point of the measurement 51 att=−1 s the load current I_(load) of the power converter 1 is measured,which in this example is 10 A. The measurement time for the subsequentmeasurements should be long enough, allowing determining the RMS valueof the load current I_(load) and/or the output voltage U_(out)accurately, consequently the measurement time should be at least onesignal period. Public networks usually have utility frequencies between50 and 60 Hz, such that the minimum measuring time for measuring theoutput voltage U_(out) for each test current should be greater than 16ms.

Then, after the measurement of the load current I_(load) at t=−1 s theload current is switched off respectively a test current I_(test) equalto zero is generated in order to measure a first open-circuit voltage 63respectively a first grid voltage U_(grid,1), after another second att=0 s. The time interval of one second is sufficiently long in order toallow the load current I_(load) stabilizing at 0 A and measuring thefirst open-circuit voltage 63 respectively a first grid voltageU_(grid,1), which is 240 V in our example (FIG. 6). After that, the testcurrent I_(test,load) is applied respectively generated by the powerconverter 1. The test current I_(test,load) should be as high aspossible to achieve reliable measurement data, but not come too close tothe nominal load current I_(nom), as the current has to be kept constantduring the measurement interval, which is not guaranteed at the nominalload current I_(nom). The test current I_(test,load) should be at least30% of the nominal load current level but not exceed 85% of the loadcurrent I_(load) before the measurement. In the present example we havechosen a test current of 80% of the load current at the start of themeasurement, respectively the test current I_(test,load) is set to 8 A.A gain, a time interval of 1 s is chosen, such that the test current hassufficient time to stabilize. The corresponding test load output voltage64 is measured at t=1 s, which is in our example U_(test,load)=247.5 V(FIG. 6).

Then the current is switched off respectively set to 0 A and afteranother second, at t=2 s, a second open-circuit voltage 65 respectivelya second grid voltage U_(grid,2) is measured (243V). The two measurementvalues permit, on one hand, to calculate an arithmetic mean of the gridvoltage U_(grid) 61 (FIG. 6, U_(grid)=241.5 V). On the other hand, it ispossible to validate the measurement by considering the variation of thegrid voltage values, which has to be small for a precise measurement.The variation between the two measurement values should be less than0.5% in reference to the nominal grid voltage. The variation of the gridvoltage curve 62 over the whole measurement interval is depicted in FIG.6. In addition it should also be verified if the test load outputvoltage 64 (U_(test,load)) is higher than the grid voltage U_(grid) 61,for instance by 1%. The maximum voltage drop U_(drop,max) due to theline impedance than can be calculated by taking the difference betweenthe test load output voltage U_(test,load) 63 and the arithmetic mean ofthe grid voltage U_(grid) und scaling it to the nominal current I_(nom):

$U_{{drop},\max} = {\left( {U_{{test},{load}} - U_{grid}} \right)\frac{I_{nom}}{I_{{test},{load}}}}$

If the grid voltage U_(grid) is submitted to a linear drift during themeasurement, the drift has no adverse effect on the exact determinationof the maximum voltage drop U_(drop,max) and therefore also on thedetermination of the sum of the line impedance 9 and the grid side lineimpedance 10, as the time intervals between the measurements are equal.

In a further embodiment of the invention the operation method includes alimiting characteristics 70 of the load current I_(load) as a functionof the output voltage U_(out) of the power converter 1 according to FIG.7, the load current I_(load) being expressed in percentages of thenominal load current I_(nom). The limiting characteristics 70 defines alimit of the load current I_(load) in function of the output voltageU_(out). Limiting the load current I_(load) in function of the outputvoltage U_(out), also has the effect of limiting the output power of thepower converter 1 in function of its output voltage U_(out). Thisfunction allows controlling the load current I_(load) in function of theoutput voltage U_(out) such that it prevents increasing the outputvoltage U_(out) due to the line impedance 9 and the grid side lineimpedance 10 over the voltage limit U_(out,lim) and prevents the powerconverter from shutting down or switching off. Consequently it alsoprevents that the voltage U_(con) at the grid connection point 6 exceedsthe maximum grid voltage U_(grid,max). Up to the first voltage level 71(U_(1000%)) the load current I_(load) is limited by the nominal loadcurrent I_(nom). The adjacent section between the first and the secondvoltage level U_(100%) and U_(0%) is linear, having a constant negativegradient 73. It starts at the first voltage level 71 (U_(100%)) with aload current I_(load) equal to the nominal load current I_(nom) and endsat the second voltage level 72 (U_(0%)) with a load current I_(load)that equals 0 A. Also above the second voltage level 72 (U_(0%)). theload current I_(load) is limited to 0 A. The first voltage level 71(U_(100%)) should be above the nominal grid voltage U_(grid), forinstance above 230 V in Europe, and below the maximum output voltageU_(out,max) of the power converter 1, for instance 282V. In thisembodiment, the gradient 73 being constant, but according to theinvention, the limiting characteristics can also be nonlinear betweenthe first voltage level 71 (U_(100%)) and the second voltage level 72(U_(0%)).

When the output voltage is above the second voltage level 72 (U_(0%)),the load current (I_(load)) is reduced to 0 A. In this case, the powerconverter 1 does not switch off and can reconnect if the output voltageU_(out) drops below the second voltage level 72 (U_(0%)).

FIG. 8 shows an illustrative numerical example of the invention, withthe first voltage level 71 at U_(100%)=240 V and the second voltagelevel 72 at U_(0%)=260V. These values may be set by the user and storedin the memory of the power converter. In addition to the currentlimiting characteristics 70, the output voltage U_(out) to load currentI_(load) characteristics 81 of the power converter 1 has been depictedin the graph in a dashed line which represents the correlation betweenthe output voltage U_(out) of the power converter 1 and the load currentI_(load) due to the line impedance between the power converter and thegrid. For no load condition, respectively for I_(out)=0 A, the outputvoltage U_(out) is equal to the grid voltage U_(grid), which is 235 V inthis example. The output voltage U_(out,nom) for the nominal loadcurrent I_(nom) is equal to 251 V.

When enabling the load current I_(load) reduction and under theassumption that the operation method to reduce the load current I_(load)as function of the output voltage U_(out) is stable, the load currentI_(load) and the output voltage U_(out) will converge towards the to thesteady state point 82 which is defined by intersection between thecurrent limiting characteristics 70 and the output voltage U_(out) toload current I_(load) characteristics 81, the steady state point havingthe coordinates (U_(limit); I_(limit)/I_(nom)).

In a possible embodiment, the output voltage U_(out) to load currentI_(load) characteristics 81 is known, respectively the grid voltageU_(grid) and/or the gradient 73, the steady state point 82 having thecoordinates (U_(limit); I_(limit)/I_(nom)) can be directly determined bythe step of performing the following calculations:

$U_{limit} = {{\frac{{U_{0\%} \cdot \left( {U_{{out},{nom}} - U_{grid}} \right)} + {U_{grid} \cdot \left( {U_{0\%} - U_{100\%}} \right)}}{U_{{out},{nom}} - U_{grid} + U_{0\%} - U_{100\%}}\text{;}\mspace{11mu} \frac{I_{limit}}{I_{nom}}} = \left( {1 - \frac{U_{limit} - U_{100\%}}{U_{0\%} - U_{100\%}}} \right)}$

Usually the grid voltage is not known such that the steady state point82 can't be calculated directly, but has to be approached step wise.

Different embodiments providing a stepwise approximation of the steadystate point 82 when the load current reduction respectively the powerreduction function is activated will be discussed below.

In one embodiment of the invention, the load current reference value isdirectly adopted for each step to the value of the current limitingcharacteristics 70 corresponding to the actual output voltage U_(out)(FIG. 9).

FIG. 9 shows the phase space diagram of the load current I_(load) andthe corresponding output voltage U_(out) of the power converter in orderto illustrate this embodiment of the operation mode. In the diagram thetransitions of the load current I_(load) and the output voltage U_(out)are plotted for different steps of adjusting the load current. The powerreduction function is activated while the power converter 1 is feeding aload current I_(load) into the grid which equals 100% of the nominalload current I_(nom), the output voltage being 251 V. The correspondingload current limit I_(load,lim) for this output voltage U_(out) is 45%of the nominal load current I_(nom). Thus a load current reference valueI_(load,ref) is generated which equals 45% of the nominal load currentI_(nom), yielding that the converter will reduce the load currentI_(load) to 45% of the nominal load current I_(nom), as shown in FIG. 9(first transition 90). Reducing the load current to 45% reduces theoutput voltage U_(out) to approximately 245 V (second transition 91).The corresponding load current limit I_(load,lim) according to thecurrent limiting characteristics 70 which corresponds to the outputvoltage U_(out)=245 V is approximately 87%. Consequently the loadcurrent reference value I_(load,ref) will be set to 87% of the nominalload current I_(nom) (third transition 92). In the following steps theload current and the output voltage U_(out) will converge towards thesteady state point 82 with the values I_(limit)=69.45 A und U_(limit)=246.11 V.

FIG. 10 shows the transient response 100 of the load current I_(load)which corresponds to the load current transitions drafted in the phasespace diagram of FIG. 9. As can be seen, the current is oscillatingaround the steady state point 82 (FIG. 9) while slowly converging to thesteady state current I_(limit), its level being indicated by a dashedline 101. The behaviour depends very much on the gradient of the loadcurrent limiting characteristics 70 and the algorithm is subject tobecome unstable if the gradient 73 of the limiting characteristics 70 isincreased.

In another embodiment, intermediate load current reference values aregenerated in contrast to the precedent example. Starting from the sameinitial condition as in the precedent example with a load currentI_(load), which equals the nominal load current I_(nom) and an outputvoltage U_(out) of 251 V, the corresponding load current limitI_(load,lim) for the first step is again 45%. In contrast to theprecedent embodiment we do not reduce the load current reference valueI_(load,ref) directly to 45% of the nominal load current I_(nom), but weset the load current reference value I_(load,ref) to an intermediatevalue between the actual load current I_(load) and the load currentlimit I_(load,lim) which corresponds to the actual voltage outputU_(out). In this embodiment we obtain the intermediate value of the loadcurrent reference value I_(load,ref) by calculating the new currentreference value by using a weighted averaging function, the load currentreference value I_(load,ref) for step n can for instance be calculatedas:

${I_{{load},{ref}}(n)} = \frac{{1 \cdot {I_{load}(n)}} + {2 \cdot {I_{{load},\lim}\left( {U_{load}(n)} \right)}}}{3}$

FIG. 11 shows the corresponding phase space diagram with the transitionsof the load current I_(load) and the output voltage U_(out) fordifferent steps. In the first step, the load current referenceI_(load,ref) will be reduced to approximately 82% of the nominal loadcurrent I_(nom) (transition 110), yielding to an output voltage U_(out)of approximately 248 V (transition 111). For this output voltage U_(out)the corresponding load current limit I_(load,lim) is 60% of the nominalload current I_(nom). The actual load current I_(load) being 82% of thenominal load current I_(nom), the new load current reference valueI_(load,ref) will be 74% of the nominal load current I_(nom) (transition112). The reduced load current I_(nom) yields to a further reduction ofthe output voltage U_(out) to approximately 247 V (transition 113) whichis already close to the steady state point 82 with the valuesI_(limit)=69.45 A und U_(limit)=246.11 V.

FIG. 12 shows the transient response 120 of the load current I_(load)which corresponds to the load current I_(load) transitions drafted inthe phase space diagram of FIG. 11. As can be seen, the load currentI_(load) is converging without overshoot to the steady state point 82,the steady state load current I_(limit) being indicated by a dashed line101. The behaviour depends very much on the gradient 73 of the loadcurrent limiting characteristics 70 and the algorithm is subject tobecome unstable if the gradient is increased.

In a further improved embodiment of the invention, the intermediate loadcurrent reference value is calculated by the formula:

$I_{{load},{ref}} = {\left( {1 - \frac{{U_{out} \cdot I_{nom}} - {U_{100\%} \cdot I_{load}}}{{U_{out} \cdot I_{nom}} + {\left( {U_{0\%} - U_{100\%}} \right) \cdot I_{load}}}} \right) \cdot I_{nom}}$

The equation is obtained when calculating the intersection point (andtherefore the steady state point 82) of the current limitingcharacteristics 70 and load current characteristics by assuming that thegrid voltage U_(grid) equals 0 V. FIG. 13 shows the transient curve 130of the load current I_(load). The load current I_(load) is convergingslowly and without overshoot towards its steady state value (I_(limit)),being indicated by a dashed line 101. The advantage of this embodimentis, that it is stable independently of the selected current limitingcharacteristics 70 and of the given output voltage U_(out) to loadcurrent I_(load) characteristics 81.

FIG. 14 shows another embodiment of the invention. In this embodimentthe first and the second voltage level are shifted, in dependency of theload current respectively with the load current dependent voltage limitU_(out,lim). The dashed line shows the load current characteristics 81,representing the dependency between the output voltage U_(out) of thepower converter 1 and the load current I_(load). FIG. 14 further showsthe limiting characteristics for two different load currents. The fullline shows the limiting characteristics 70 a for a load current I_(load)which is 100% of the nominal current I_(nom) while the dotted line showsthe limiting characteristics 70 b for a load current which is 50% of thenominal current I_(nom). The limiting characteristics 70 a, 70 b areshifted in function of the load current I_(load), respectively theirfirst voltage levels 71 a, 71 b (U_(100%), U′_(100%)) and their secondvoltage levels 72 a, 72 b (U_(0%), U′_(0%)) are shifted in dependency ofthe voltage limit U_(out,lim). In this example the first voltage levelU_(100%) is chosen that it coincides with the output voltage U_(out,nom)of the inverter at the nominal current I_(nom) at the maximum allowedgrid voltage U_(max,grid). The intersection point 82 a of the loadcurrent characteristics with the limiting characteristics for nominalload current I_(nom) has the coordinates (U_(100%)(I_(nom));I_(nom)/I_(nom)). The second voltage limit 72 a is chosen that thegradient 73 of the voltage limiting characteristics is steeper than thegradient of the load current I_(load) characteristics 81. Such, that inthe case that the grid voltage U_(grid) increases over the maximumallowed grid voltage U_(max,grid), the load current I_(load) will bereduced faster, preventing an overvoltage and a switch off of the powerinverter 1. The load current will be reduced fast enough, such that thereduction of the output voltage U_(out) due to the reduced load currentI_(load) will be greater than the reduction of the voltage limitU_(out,lim), which is also linked to the load current I_(load).

With a decreasing/increasing load current I_(load) the first voltage andthe second voltage level are shifted towards a lower/higher voltage. Inthe example of FIG. 14, both voltage levels are shifted by the sameamount ΔU such that at a load current I_(load) of 0 A the second voltagelevel U_(0%) coincides with the maximum allowed grid voltageU_(grid,max):

${\Delta \; U} = {\left( {U_{{out},\max} - U_{{grid},\max}} \right) \cdot \frac{I_{load}}{I_{nom}}}$

In the present example the first voltage level U_(100%) is shifted by−13.5V from the first voltage level 71 a to another first voltage level71 b when decreasing the load current to 50% of the nominal load currentI_(nom). Similarly the second voltage level U_(0%) is shifted by −13.5Vfrom the second voltage level 72 a to another second voltage level 72 b.

The intersection point 82 a for the nominal load current I_(nom) withthe coordinates (273V; 100%) is moved to a new intersection point 82 bhaving the coordinates (263V; 50%). With an increasing load current, thefirst and the second voltage level will move towards a higher voltage.

FIG. 15 shows schematically a power converter 1 according to theinvention, feeding a three phase load current I_(load) into a grid 3. Inthis embodiment, the power converter 1 is fed by a DC voltage source 12,for instance a photovoltaic system, which is connected to the input 8 ofthe power converter 1. The power converter 1 outputs a three phase loadcurrent I_(load) into the grid 3, the load current I_(load) being an ACcurrent. The line, where the load current I_(load) is fed into the gridcan be split in two parts: a dedicated three-pole lead wire 4, whichconnects the output 5 of the power converter 1 to the grid connectionpoint 6 and a grid side connection network 7, which connects the gridconnection point 6 to the grid 3. The line impedance of the three-polelead wire can be represented by a lumped three-phase line impedance 9,the line impedance of the grid side connection network 7 by a lumpedthree phase grid side line impedance 10.

The power converter 1 comprises a three phase bridge 140 implementedusing six high speed switches, typically IGBT or MOSFETs, which isconnected to the input 8 to the power converter. The switches arepulse-width-modulated (PWM) to impress the load current I_(load) via afilter module 144 into the grid 3. The power converter further comprisesa current controller 141 which controls the switches impressing thecurrents into the grid 3. The current controller 141 is connected tothree load current sensors 142, preferably hall-effect-sensors, each ofthem measuring the load current I_(load) of one the three phasesconnecting the power converter 1 to the grid at the grid connectionpoint 6. The current controller is also connected to three outputvoltage sensors 143, each of them connected between two phases of thethree phase lead wires 4. The current controller 141 preferablycomprises a micro-controller which controls the switches of the threephase bridge 140.

In summary, it is to be noted that the invention creates a method foroperating a power converter feeding a load current into a grid, whichenables to compensate the influence of the line impedance between theoutput of the power converter and the grid. By adjusting the voltagelimit to the load current it allows the efficient use of the availableoutput power of the converter while avoiding additional hardware costs.

It is also to be noted that, with the load current reduction in functionof the output voltage, an operation method is provided. It allows acontinuous operation of the power converter even when operating with anoutput voltage close to maximum limits as for example the maximum outputvoltage or the voltage limit. By preventing that the power converterwill be switched off and has to be reconnected to the grid, the powerconverter can work more reliably and more efficiently.

1. Method for operating a power converter that delivers output currentinto a grid, including the steps of a) determining the output current ofthe power converter; b) monitoring an output voltage of the powerconverter; c) controlling the output current in order to prevent theoutput voltage from exceeding an output voltage limit, whereincontrolling the output current includes the step of defining a limitingcharacteristics of the output current in dependency of the outputvoltage, the limiting characteristics i) being equal to the nominaloutput current, below a first output voltage level, ii) having anegative gradient between a first output voltage level and a secondoutput voltage level, iii) covering an output current value rangebetween zero and the nominal output current and iv) being zero above thesecond output voltage level, v) wherein the first output voltage levelis smaller than the second output voltage level and vi) the secondoutput voltage level being equal or smaller than a maximum outputvoltage.
 2. Method according to claim 1, wherein the limitingcharacteristics between the first output voltage level and the secondoutput voltage level is chosen to be linear.
 3. Method according toclaim 1 wherein the negative gradient between the first output voltagelevel and the second output voltage level is variable.
 4. Methodaccording to claim 3 wherein the limiting characteristics has acontinuous negative gradient, preferably with piecewise constantgradients.
 5. Method according to claim 2, wherein controlling theoutput current includes the step of adjusting the output currentstepwise to meet the limiting characteristics by generating intermediateoutput current reference values.
 6. Method according to claim 5, whereinthe intermediate output current reference values I_(load,ref) aredetermined by the formula:${I_{{load},{ref}} = {\left( {1 - \frac{{U_{out} \cdot I_{nom}} - {U_{100\%} \cdot I_{load}}}{{U_{out} \cdot I_{nom}} + {\left( {U_{0\%} - U_{100\%}} \right) \cdot I_{load}}}} \right) \cdot I_{nom}}},$U_(100%) being the first output voltage level, U_(0%) being the secondoutput voltage level, I_(load) being the actual output current, U_(out)the actual output voltage, and I_(nom) the nominal output current of thepower converter.
 7. Method according to claim 1, including the step ofmeasuring the grid voltage, calculating a steady state output current,generating an output current reference value equal to the steady stateoutput current and controlling the output current to reach the steadystate output current.
 8. Method according to claim 1, wherein the outputvoltage limit is adjusted in dependency of the output current.
 9. Methodaccording to claim 8, wherein the first output voltage level and thesecond output voltage level are adapted in dependency of the voltagelimit.
 10. Method according to claim 8, wherein the first output voltagelevel and the second output voltage level are kept constant independency of the voltage limit.
 11. Method according to claim 8,wherein the first output voltage level and the second output voltagelevel are shifted by different values in dependency of the voltagelimit.
 12. Method according to claim 1, wherein a timer function shutsdown the power converter if the output voltage exceeds the second outputvoltage level for a predetermined time.
 13. Method according to claim 1,wherein feedback control is applied to the output current based on acurrent reference.
 14. Method for operating a power converter having anoutput for feeding a load current into a grid, including the steps of a)determining the load current at the output of the power converter, b)monitoring an output voltage of the power converter; c) controlling theload current to avoid that the output voltage exceeds an output voltagelimit, wherein controlling the load current includes the step ofdefining a limiting characteristics of the load current in dependency ofthe output voltage, the limiting characteristics i) being equal to thenominal load current, below a first output voltage level, ii) having anegative gradient between a first output voltage level and a secondoutput voltage level, iii) covering a load current value range betweenzero and the nominal load current and iv) being zero above the secondoutput voltage level, v) wherein the first output voltage level issmaller than the second output voltage level and vi) the second outputvoltage level being equal or smaller than a maximum output voltage. 15.A power converter having an output for connecting to a grid, includingan output current sensor, an output voltage sensor and an output currentcontroller, where the power converter is adapted to feed an outputcurrent into a grid by: a) determining the output current of the powerconverter, b) monitoring the output voltage of the power converter, a)controlling the output current in order to prevent the output voltagefrom exceeding an output voltage limit, wherein controlling the outputcurrent includes the step of defining a limiting characteristics of theoutput current in dependency of the output voltage, i) the limitingcharacteristics being equal to the nominal output current, below a firstoutput voltage level, ii) having a negative gradient between a firstoutput voltage level and a second output voltage level, iii) covering anoutput current value range between zero and the nominal output currentand iv) being zero above the second output voltage level, v) wherein thefirst output voltage level is smaller than the second output voltagelevel and vi) the second output voltage level being equal or smallerthan a maximum output voltage.
 16. Method for operating a powerconverter having an output for feeding a load current into a grid,including the steps of a) determining the load current at the output ofthe power converter, b) monitoring an output voltage of the powerconverter, c) controlling the load current, to avoid that the outputvoltage exceeds a voltage limit, wherein controlling the load currentincludes the step of defining a limiting characteristics of the loadcurrent in dependency of the output voltage, the limitingcharacteristics i) being equal to the nominal load current, below afirst output voltage level, ii) having a negative gradient between afirst output voltage level and a second output voltage level, iii)covering a load current value range between zero and the nominal loadcurrent, iv) being zero above the second output voltage level, v)wherein the first output voltage level being smaller than the secondoutput voltage level, vi) the second output voltage level being equal orsmaller than the maximum output voltage.